A liquid crystal display (LCD) which is one typical thin panel has been widely used for a personal computer, a monitor for a personal digital assistant, and the like by utilizing its advantages of being low in power consumption, small in size, and light in weight. In recent years, the liquid crystal display has been widely used for a TV monitor and the like.
In particular, an active matrix substrate (referred to hereinafter as a “TFT substrate”) including thin film transistors (TFTs) used as switching elements is well known as a substrate for use in an electro-optic device such as a LCD. A LCD (referred to hereinafter as a TFT-LCD) employing a TFT substrate has been required to achieve not only improvements in display performance such as a wider viewing angle, higher definition, and higher quality but also reductions in costs resulting from simplified manufacturing steps and efficient manufacture.
A typical TFT-LCD includes a TFT substrate which is an element substrate including a plurality of pixels disposed in a matrix and each including a pixel electrode and a TFT connected thereto, and a CF substrate which is a counter substrate including a counter electrode opposed to the pixel electrodes, a color filter (CF), and the like. A liquid crystal cell configured such that a liquid crystal layer is held between these substrates is used as a basic structure. Polarizers and the like are mounted to the liquid crystal cell. A full transmission type LCD, for example, includes a backlight (BL) provided on the back surface side of the liquid crystal cell.
An example of the liquid crystal cell in which the pixel electrodes for generating an electric field for driving liquid crystal and the counter electrode are disposed so as to hold the liquid crystal layer therebetween in this manner includes a vertical electric field driving mode liquid crystal cell typified by a TN (Twisted Nematic) mode. In general, TN mode TFT substrates are often manufactured by undergoing five photolithographic steps (photolithographic processes), as disclosed in Patent Document 1 to be described below which illustrates a manufacturing method, for example. These structures are based on TFTs (BCE type TFTs) employing a back channel etching (BCE) structure as a basis.
On the other hand, an IPS (registered trademark) (In Plane Switching) mode which is a horizontal electric field driving mode in which both the pixel electrodes and the counter electrode are disposed on the TFT substrate has been proposed from the viewpoint of widening the viewing angle of the TFT-LCD. The IPS mode provides a wider viewing angle than the vertical electric field driving mode, but finds difficulties in providing bright display properties because of the problem that an image display portion in the IPS mode has a lower aperture ratio and a lower transmittance than that in the vertical electric field driving mode. This problem results from the fact that an electric field for driving liquid crystal does not effectively act on the liquid crystal lying in an area immediately over comb tooth shaped pixel electrodes. As a horizontal electric field driving mode capable of mending this problem, a fringe field switching (FFS) mode as disclosed in Patent Document 2, for example, has been proposed.
In the FFS mode, the counter electrode and the pixel electrodes are disposed on the TFT substrate, with an interlayer insulation film therebetween. One of the counter electrode and each pixel electrode which is provided as an upper layer has liquid crystal controlling slits (or a comb tooth shape) formed therein and serves as a slit electrode (or a comb tooth electrode). Such a FFS mode enables a horizontal electric field to be applied to liquid crystal molecules lying immediately over a pixel portion by generating an oblique electric field (fringe field), thereby sufficiently driving the liquid crystal molecules. This provides a wide viewing angle and a higher transmittance than in the IPS mode. In this FFS mode configuration, a reduction in pixel aperture ratio is prevented by forming the pixel electrodes, the counter electrode, and the liquid crystal controlling slit electrode made of a transparent conductive film. Unlike the TN mode LCDs, it is not always necessary for the FFS mode LCDs to separately provide a storage capacitor pattern in pixels because a pixel electrode and the counter electrode form a storage capacitor. From these viewpoints, the FFS mode configuration achieves a liquid crystal display having a high pixel aperture ratio.
In general, amorphous silicon (a-Si) has been used for a semiconductor channel layer in switching elements of TFT substrates for conventional LCDs. A principal reason therefor is that a film having uniform properties is formed on a substrate having a large area because of its amorphousness. Another principal reason is that an amorphous silicon film, which can be deposited at a relatively low temperature, can be manufactured on a less heat-resistant and less expensive glass substrate to consequently have good compatibility with displays for liquid crystal display devices for typical TVs.
In recent years, TFTs (oxide semiconductor TFTs) in which an oxide semiconductor is used for a channel layer have been frequently developed (for example, Patent Documents 3 and 4, and Non-Patent Document 1). Examples of the oxide semiconductor include zinc oxide (ZnO) based semiconductors and InGaZnO based semiconductors prepared by adding gallium oxide (Ga2O3) and indium oxide (In2O3) to zinc oxide (ZnO). These oxide semiconductor films have higher permeability to light than Si semiconductor films. For example, an oxide semiconductor film having a permeability of not less than 70% to visible light in the range of 400 to 800 nm is disclosed in Patent Document 5.
The oxide semiconductor is advantageous not only in providing an amorphous film having good uniformity with stability by making the composition thereof proper but also in achieving small-sized high-performance TFTs because of its higher mobility than conventional a-Si. This is advantageous in that a TFT substrate having a high pixel aperture ratio is provided by applying such an oxide semiconductor film to the TFTs of pixels. Thus, the use of oxide semiconductor TFTs for a FFS mode TFT substrate achieves LCDs having both a wider viewing angle and brighter display properties.
Further, it has been necessary that circuits having a relatively large area as driving circuits for applying a driving voltage to pixel TFTs are mounted to the TFT substrate because a-Si is relatively low in mobility. However, the driving circuits employing oxide semiconductor TFTs having high mobility are achieved by circuits having a relatively small area, so that the driving circuits can be produced on the same substrate as the pixel TFTs. This is advantageous in eliminating the need to individually mount the driving circuits to thereby produce a LCD at low costs, and in providing a narrower frame region for a LCD which has been required in space for mounting the driving circuits.
However, the oxide semiconductor is in general poor in chemical resistance to have the property of easily dissolving in a weakly acid chemical liquid such as oxalic acid (carboxylic acid). Thus, if the oxide semiconductor is used for TFTs having a BCE structure which go mainstream for a-Si and source and drain electrodes immediately over a channel layer are formed by wet etching using an acid chemical liquid, there arises a problem that the oxide semiconductor for the channel layer is also etched. As a result, there has been a problem that a Channel region with high reliability cannot be formed.
To solve this problem, a TFT having an etch stopper (ES) structure in which a protective insulation film is formed on a channel region of an oxide semiconductor, for example, as disclosed in Patent Document 6 and a TFT having an inverted coplanar structure disclosed in Patent Documents 7, 8, and 9, and the like have been proposed. In the ES structure, the protective insulation film prevents the channel region from being exposed to a chemical liquid during the process of wet etching of the source and drain electrodes. In the inverted coplanar structure, the channel region is prevented from being exposed to a chemical liquid because an oxide semiconductor having a channel region is formed after the formation of the source and drain electrodes. Thus, oxide semiconductor TFTs with high reliability are produced. It should be noted that the inverted coplanar structure disclosed in Patent Documents 7, 8, and 9 is used as a structure for preventing process damages also in organic semiconductor TFTs in which organic compounds are used for a semiconductor channel layer (for example, Patent Document 10).